The present invention is directed to differential signaling devices, and especially to multi-stage differential signaling output devices. The present invention is particularly useful with low voltage multi-stage differential signaling output devices.
Apparatuses designed to perform as interface drivers for use with LVDS (low voltage differential signaling) output stages require precise common mode control for the output signal. Such LVDS output stage devices are often embodied in multi-stage configurations involving multiple differential transistor pairs in a serial cascaded arrangement. Each differential transistor pair has level shifter or follower transistors used to adjust the common mode for a succeeding stage. To regulate the common mode of the final output stage, a sample of the output signal is provided to an error amplifier, and compared with a reference voltage to generate a feedback signal to effect the required feedback control. Such an arrangement provides compensation for temperature variation. As the base-to-emitter voltage (Vbe) of the follower transistors varies over temperature, that change affects the sample of the output signal provided to the error amplifier that provides the feedback signal for regulating the common mode of the final output stage.
The prior art topology described above for regulating voltages in differential signaling output devices has been useful in prior art devices, but is problematic in low voltage output devices. In low voltage differential signaling output devices all stages previous to the output stage require some common mode regulation to prevent saturating the differential transistor pair in the output stage. Prior art differential signaling output devices provide a regulator circuit for each stage preceding the final output stage to regulate the common mode for each stage and avoid saturating the differential transistor pair in the next succeeding stage. Simply reducing each interstage supply voltage provided t a succeeding stage using a resistor ignores the effects of variations in temperature and variations in supply voltage.
Providing a substantially duplicate regulator circuit for each stage requires relatively large sampling resistors that generate heat and require significant areas of silicon to implement. However, simply eliminating interstage sense resistors and regulating interstage supplies to a fixed voltage results in large variations in common mode voltage as temperature varies because no Vbe temperature compensation is provided.
There is a need for an apparatus for regulating voltage for at least one differential transistor pair having a voltage follower buffer exhibiting a voltage-temperature response.
There is a need for an apparatus for providing a regulated voltage signal to selected stages of a multi-stage differential signaling device, the selected stages each having a voltage follower buffer exhibiting a voltage-temperature response.